REVENUES (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 3,144,097	$ 3,412,190	$ 3,300,303
Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	1,062,714	1,120,043	1,037,388
Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	461,036	340,906	304,727
Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	480,621	632,550	662,833
Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	809,236	851,520	795,077
Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	330,490	467,171	500,278
United States
Disaggregation of Revenue [Line Items]
Total revenues	2,739,557	2,940,832	2,796,145
Canada
Disaggregation of Revenue [Line Items]
Total revenues	404,540	471,358	504,158
Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	2,137,306	2,237,068	2,141,194
Safety-Kleen
Disaggregation of Revenue [Line Items]
Total revenues	1,010,202	1,178,129	1,161,282
Operating segments | Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	2,004,081	2,092,363	2,003,843
Operating segments | Environmental Services | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	1,062,714	1,120,043	1,037,388
Operating segments | Environmental Services | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	461,036	340,906	304,727
Operating segments | Environmental Services | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	480,331	631,414	661,728
Operating segments | Environmental Services | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Operating segments | Environmental Services | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Operating segments | Environmental Services | Oil, Gas and Lodging Services and Other
Disaggregation of Revenue [Line Items]
Total revenues		117,000	119,800
Operating segments | Environmental Services | United States
Disaggregation of Revenue [Line Items]
Total revenues	1,690,256	1,721,322	1,598,402
Operating segments | Environmental Services | Canada
Disaggregation of Revenue [Line Items]
Total revenues	313,825	371,041	405,441
Operating segments | Safety-Kleen
Disaggregation of Revenue [Line Items]
Total revenues	1,139,726	1,318,691	1,295,355
Operating segments | Safety-Kleen | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Operating segments | Safety-Kleen | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Operating segments | Safety-Kleen | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Operating segments | Safety-Kleen | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	809,236	851,520	795,077
Operating segments | Safety-Kleen | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	330,490	467,171	500,278
Operating segments | Safety-Kleen | United States
Disaggregation of Revenue [Line Items]
Total revenues	1,049,975	1,220,096	1,196,661
Operating segments | Safety-Kleen | Canada
Disaggregation of Revenue [Line Items]
Total revenues	89,751	98,595	98,694
Corporate, non-segment
Disaggregation of Revenue [Line Items]
Total revenues	290	1,136	1,105
Corporate, non-segment | Technical Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Corporate, non-segment | Field and Emergency Response Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Corporate, non-segment | Industrial Services and Other
Disaggregation of Revenue [Line Items]
Total revenues	290	1,136	1,105
Corporate, non-segment | Safety-Kleen Environmental Services
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Corporate, non-segment | Safety-Kleen Oil
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0
Corporate, non-segment | Oil, Gas and Lodging Services and Other
Disaggregation of Revenue [Line Items]
Total revenues		1,100	1,100
Corporate, non-segment | United States
Disaggregation of Revenue [Line Items]
Total revenues	(674)	(586)	1,082
Corporate, non-segment | Canada
Disaggregation of Revenue [Line Items]
Total revenues	$ 964	$ 1,722	$ 23